1. Field of the Invention
This invention relates to a magnetic recording medium and a process for production thereof.
2. Description of the Prior Art
At present, magnetic tapes have become employed extensively in audio, video and computer fields. Accordingly, the amount of information to be recorded in media has showed a yearly increase, and a higher recording density has thus been increasingly required of media.
In the current recording method utilizing a magnetic head, the spacing loss between tape--head is expressed as 54.6 d.lambda. [dB] (d: the tape--head distance, .lambda.: the recording wavelength). As can be seen from this formula, in a short wavelength recording with a high recording density, the extent of the reduction in output by spacing is significantly greater than that of a longer wavelength. Therefore, if even a small foreign matter exists on the tape surface, it is inevitably detected as a dropout.
As the possible causes for dropouts, there may be contemplated the fall-off of the magnetic powder from the magnetic tape coated surface resulting from the deterioration of the coat due to repeated applications of stress, the scrape-off of the base during running, dust etc. first statically adhered to the base surface and then further transferred to the coat surface, and so forth. In order to prevent these, for example, for the latter cause, such methods were designed as that which reduces electrification of the base by coating a paint of carbon black, graphite etc. kneaded with an organic binder, or by coating an antistatic agent, or the like, that which aims to toughen the base and hence reduce the scrape-off of the base by coating a paint of silicon dioxide etc. kneaded with an organic binder, and so forth. These treatments can considerably depress the tendency of the increase in dropouts upon repeated runs. However, its level cannot be said perfectly satisfactory under the existing circumstances, and further reduction is required.
As the result of the detailed investivations into the cause for the generation of dropouts in order to further reduce them, the following have been made clear. In forming a back surface, if the back surface has been formed before the magnetic surface, then on surface smoothening by the calandering treatment subsequent to the formation of the magnetic surface, the unevenness of the back surface is transferred to the magnetic surface, and smoothening of the magnetic coat is therefore not satisfactorily achieved. For that reason, the back coat treatment is generally conducted, after a magnetic coat has been formed on a substrate, on the other side of the substrate. Since the back layer is required to be tough so that dropouts are not increased with the increase in number of runs, a thermosetting resin is generally employed as a binder. In that case, the back layer is coated, then the tape is wound up, and the thermosetting treatment is applied. However, just after the coating has been finished, a curing reaction has not started yet in the back layer and hence the coat is weak and moreover the back surface and the magnetic surface are in intimate contact with each other, the surface of the back surface coat containing carbon black, graphite or other inorganic filler filled in the back layer coat tends to be easily transferred to the opposite magnetic layer surface in intimate contact therewith; that thus transferred was found to be a cause for dropouts, plugging of the head etc. It is also presumed that a phenomenon silimar to such a phenomenon could occur with a thermoplastic resin. This accounts for that by providing a back layer, although the increase in dropouts can be depressed by repeated runs, the dropouts are not so reduced as expected in the stage of a small number of runs.